The present invention relates to a phase modulation apparatus and a phase modulation method for performing a phase modulation on an optical signal at a high speed. For example, the invention relates to a high-speed modulation apparatus and its method for quantum cryptography of phase modulation system.
FIGS. 14 and 15 show a method for phase modulation in the conventional quantum cryptography of phase modulation system disclosed in a document: Uchiyama, xe2x80x9cBasic quantum mechanics and quantum cryptographyxe2x80x9d Mathematical Sciences No. 402, December 1996.
According to the cryptography of phase modulation system, a phase modulator (PM) is usually placed as shown in a configuration of FIG. 14. A transmitting apparatus and a receiving apparatus include phase modulators (PMA, PMB), respectively, and apply voltage corresponding to the phase modulation (0, xcfx80/2, xcfx80, 3xcfx80/2) to the phase modulators.
The quantum cryptography of phase modulation system physically creates the system utilizing interference of photons having different phases which flow through two kinds of optical paths having the same length in an optical system using such as an optical fiber. In the modulation of the quantum cryptography of phase modulation system, each of the transmitting apparatus and the receiving apparatus usually includes one phase modulator and operate phase modulation between the two partners.
The following explains concretely the conventional method for phase modulation in the quantum cryptography of phase modulation system using one example.
For example, an embodiment of B92 protocol using a coupler at a joint where an optical fiber is used will be explained referring to FIGS. 14 and 15.
A photon is oscillated from laser at a certain cyclic frequency, and transmits to a coupler 1 as an optical signal through the optical fiber, etc. Hereinafter, only two optical paths will be explained, which are critical to the quantum cryptography.
The first one is a first optical path in which, at a coupler 1, the laser goes upwards in FIG. 14, passes through the phase modulator PMA and a coupler 2, and at a coupler 3, the laser goes forward without passing through a phase modulator PMB and passes through a coupler 4 to reach a detector.
The second one is a second optical path in which at the coupler 1, the laser goes forward without passing through the phase modulator PMA and passes through the coupler 2, and at the coupler 3, the laser goes upwards in FIG. 14, passes through the phase modulator PMB, passes through the coupler 4 to reach the detector.
The first optical path has the same length as the second optical path.
When it is assumed that one phase as "PHgr"a which modulated by the phase modulator PMA, and the other as "PHgr"b which modulated by the phase modulator PMB, between the optical paths having the same length, an optical interference occurs according to a difference value of ("PHgr"axe2x88x92"PHgr"b). The quantum cryptography of the phase modulation system utilizes this interference. As described, the conventional method for modulating the phase simply places one phase modulator in each of the transmitting device and the receiving device to perform desired phase modulation.
For example, in order to modulate phases of 0, xcfx80/2, xcfx80, or 3xcfx80/2, voltages of 0V, 4V, 8V, or 12V are applied to the phase modulator. As shown in FIG. 15, upon applying the voltage, there always exist a rise time LP and a fall time TP of the voltage. Further, when one phase modulator continuously modulates different random phases serially, a blank period BP (BP greater than 0) should be provided.
In the conventional phase modulation method, the modulation requires to apply voltages in different values at a high speed, however, the rise time LP and the fall time TP of voltage prevents a high-speed operation. For example, when a photon is oscillated at a certain cyclic frequency using a pulse laser, the phase should be modulated synchronously with the timing of oscillating the photon, and therefore, various kinds of voltages corresponding to the modulation amount should be applied synchronously with the above timing. A bit rate of the quantum cryptography is in proportion with the number of frequency of the pulse laser, so that the voltage should be switched to a high voltage and applied to the phase modulator to perform the high-speed operation. Because of this, generally, long LP and TP cause to extend the time required for applying the voltage different from the previous value. Ignoring this fact, if the cyclic frequency (of the pulse laser, for example) is raised, BP becomes hard to be taken (BPxe2x89xa60), the next timing of the voltage to rise and the previous timing of the voltage to fall are overlapped, which causes malfunction of the modulator and, therefore, prevents the high-speed operation.
The present invention aims to perform a high-speed processing of the phase modulation. The objectives of the invention are to improve a bit rate, namely, the high-speed operation in, for example, the quantum cryptography of the phase interference system.
According to the present invention, a phase modulation apparatus includes:
N phase modulators (N: an integer equal to or greater than 2) for performing a phase modulation on an optical signal which flows through an optical path;
a first optical switch and a second optical switch for selectively connecting one of the N phase modulators which are placed parallel to the optical path; and
a control unit for outputting a switching signal to the first optical switch and the second optical switch and making the one of the N phase modulators connect to the optical path.
The control unit includes:
a phase modulation data memory for storing phase modulation data;
N voltage generating units each of which generates a voltage corresponding to each of the phase modulation to the N phase modulators; and
a switching unit for serially reading the phase modulation data stored in the phase modulation data memory and serially supplying the phase modulation data read to the voltage generating unit for generating the voltage to the phase modulator to be connected to the optical path by the first optical switch and the second optical switch.
The control unit includes:
a phase modulation data memory for storing phase modulation data having N values;
a voltage generating unit for generating N fixed voltages corresponding to the N values of the phase modulation data, and for supplying the N fixed voltages to the N phase modulators, respectively; and
a switching unit for reading the phase modulation data stored in the phase modulation data memory, and outputting the switching signal for selecting one of the N phase modulators, to which the voltage corresponding to a value of the phase modulation data read is supplied, to the first optical switch and the second optical switch.
According to the present invention, a phase modulation apparatus includes:
N phase modulators (N: an integer equal to or greater than 2) which are connected serially to an optical path; and
a control unit for generating N voltages to the N phase modulators so that a sum of the N voltages applied to the N phase modulators becomes a voltage corresponding to a phase modulation of an optical signal, and supplying the N voltages to the N phase modulators.
The control unit includes:
a phase modulation data memory for storing phase modulation data; and
a voltage generating unit for generating N equal voltages (V/N) by equally dividing a voltage V, which corresponds to the phase modulation data stored in the phase modulation data memory, by N, and supplying the N equal voltages (V/N) generated to the N phase modulators, respectively.
According to the present invention, a phase modulation method for serially performing a phase modulation on a first optical signal and a second optical signal, the phase modulation method includes:
a first modulation step for phase modulating the first optical signal by a first phase modulator; and
a second modulation step for phase modulating the second optical signal by a second phase modulator before the first modulation step terminates.
According to the present invention, a phase modulation method for performing a phase modulation on an optical signal by a predetermined amount includes:
a first modulation step for phase modulating the optical signal by a first phase modulator; and
a second modulation step for phase modulating the optical signal, which is phase modulated by the first modulation step, by a second phase modulator, and
the phase modulation method performs the phase modulation on the optical signal by the predetermined amount with performing both the first modulation step and the second modulation step.